Such processes are for example suitable to measure distances, magnetic fields, pressure differences etc. when the characteristic output curve of the sensor used for the measurement is not linear and is either unknown at the beginning or unstable, so that in the latter case, although originally known, it has changed in the meantime as a function of time, temperature or some other factor in an unknown manner so that it is again unknown and must be recalibrated. In this process values of a function f[x] of the physical variable x are measured. However not the values of the function f[x] but the values x=f.sup.-1 [x] of the physical variable are desired, and this is only possible if the characteristic output curve of the sensor used is known and calibrates or recalibrates itself automatically.
It is known that in conventional non-automatic calibrations of a sensor used for measurements, the corresponding values of the output signal f[x] of the sensor are determined point by point for each value of the physical variable x to be measured by means of a measuring device that is as precise as possible. Re-calibrations or calibrations are then generally required from time to time, for example at fixed time intervals.
An automatic calibration process is known from the published document "Applied Optics, Feb. 1, 1989, Vol. 28, No. 3, pages 419 to 420, Loss compensated fiber-optic displacement sensor including a lens, E. Bois, S. J. Huard and G. Boisde", in which a self-calibrating and loss-compensating optical distance sensor is described. The distance sensor contains two light conductors for the purpose of self-calibration, the first ends of which are axially offset in relation to each other and are arranged facing a light-reflecting surface of a mobile body moving along the optical axis of the light conductors. The second end of each light conductor is alternately connected to a light diode or to a photo diode. Each light diode emits light and is excited alternately in order to evaluate by means of a microcomputer two signals detected thereupon by the photo-diodes The light emitted reaches the light-reflecting surface via the appertaining light conductor and the reflected light then reaches via the other light conductor one of the photo-diodes whose output signals are transmitted to the microcomputer. The microcomputer in turn holds the reference signal transmitted by the light diode constant via a digital/analog converter and calibrates the measuring arrangement by utilizing the relationship between the sum and the difference of the two detected signals. The relationship is near the zero point proportional to the desired value of the distance when the presence of a parabolic characteristic output curve can be assumed for the sensors.
It is the object of the instant invention to provide a process of the type mentioned initially by using two nearly identical sensors making it possible to calibrate or recalibrate the output curve of a first one of the two sensors by using the measuring results of the second sensor without rigorous requirements as far as the configuration of the output curve of the sensors is concerned and without any other overly restrictive conditions.